The invention relates to a process for the production of a can comprising a base and a tubular body from sheet metal which is coated on at least one side with a layer of plastic, in which process, firstly, a round disc is produced from the sheet metal, which disc is then deep-drawn into a cup which is coated at least on the outside with the layer of plastic, after which this cup is formed into a can by wall ironing, the wall ironing taking place in a single stroke by moving the cup successively through a plurality of wall-ironing rings. A process of this nature is described in European Patent No. 0,402,006 B1, which is based on a laminate comprising an aluminium sheet. This patent proposes that the problems with processing of this laminate be solved by employing a combination of a proposed exit angle from a wall-ironing ring and an entry angle thereof which is selected between 1 and 4xc2x0. This patent also proposes a specific selection of material for the wall-ironing ring.
U.S. Pat. No. 3,765,206 proposes the wall ironing of cans from coated steel utilizing a single wall-ironing ring with an entry angle of between 4 and 6xc2x0. In this case, the entry angle is understood to mean the angle which the plane of entry to a wall-ironing ring forms with the axis of the said ring. However, this document relates only to steel sheet with a metallic coating.
EP patent application no. 0298560A2 describes a method of wall ironing in two die regions in which in one die region hydrodynamic lubrication under pressure is applied, whereas in the other die region there is no lubrication.
It has been found that various problems may arise with wall ironing for the production of a can from a laminate based on a steel sheet and a layer of plastic. Some of these problems relate to the layer of plastic. During the deep drawing to form a cup, this layer of plastic may begin to form loose strands, may acquire a rough surface or may even rupture entirely. However, problems may also be caused by the fact that the expansion force in the wall-ironing rings is excessive, and this may lead to excess wear to these rings, to dimensional inaccuracy of the product or even to these rings fracturing. In general, the expansion force in a wall-ironing ring will increase as the entry angle selected becomes smaller.
It has been found that using the invention enables these problems to be drastically reduced.
The invention therefore consists in the fact that, when a sheet metal is used which has been selected from the group comprising steel sheet, tin coated steel sheet (tinplate) and chromiumxe2x80x94chromium oxide coated steel sheet (ECCS), the entry angle for each of the successive at least three wall-ironing rings is smaller than that of the preceding ring. It has been found that an entry angle for the first wall-ironing ring should be relatively large in order to prevent the expansion force in this first ring becoming excessive. However, in the following rings the entry angle should become smaller in order to prevent the surface of the layer of plastic from becoming rough.
Good results can be obtained if three wall-ironing rings are used, with the ratio between the entry angles for the first wall-ironing ring and the second wall-ironing ring lying between 1.3 and 3.0 and the ratio between the entry angles for the second wall-ironing ring and the third wall-ironing ring lying between 1.4 and 2.8. Preferably, the said ratios between the entry angles are selected to lie between 1.7 and 2.4, and between 1.7 and 2.3, respectively.
Tests have shown that the optimum entry angle for the first wall-ironing ring is partly dependent on the speed at which the can is formed. This speed is often expressed as the number of can production strokes C per minute. An optimum entry angle for the first wall-ironing ring is then A:Cxc2x0, where A is selected to be between 560 and 1280 and C represents the number of can production strokes per minute.
During wall ironing, the metal base and the layer of plastic simultaneously undergo considerable deformation. It is important that the layer of plastic continues to form a smooth and continuous surface which adheres well to the metal. Examinations carried out using various plastics in the novel process have shown that after undergoing considerable deformation different plastics may exhibit considerable differences in the extent to which they are crystallized. An indication of the level of crystallization of a polymeric material is obtained by an X-ray diffraction measurement of this plastic. This diffraction measurement measures the extent to which chain molecules of the polymer, or parts of these molecules, are oriented with respect to one another. This measurement technique is generally known and therefore does not require further explanation here. A description of this measurement method is given in xe2x80x9cGxc3xcnther Kxc3xa4mpf; Characterization of Plastics by Physical Methods, Hanser Publishers, page 101xe2x80x9d. It has been found that it is preferable, in the novel process, to use as the layer of plastic a material which is able to crystallize to a considerable extent as a result of deformation. This reduces the risk of the layer of plastic being damaged or torn off the metal sheet during the wall ironing. In particular, it is preferred to use a plastic of which the maximum crystallinity after wall ironing, as determined by X-ray diffraction measurement, is at least 20%.
In this context, a plastic which has proven eminently suitable is a polyethylene terephthalate with a melting point of higher that 240xc2x0 C. and an intrinsic viscosity of higher than 0.6, if it is applied to the steel sheet in a layer thickness of between 15 and 30 xcexcm.
It should be noted that it is possible to determine in the following way whether a layer of plastic crystallizes as a result of deformation sufficiently to be suitable as a coating for the outside of a plastic-coated can as produced in accordance with the novel process.
A layer of amorphous plastic with a thickness of approx. 30 xcexcm is applied to one side of an ECCS strip, with a suitable thickness of, for example, 0.26 mm, by means of lamination or extrusion coating. The coated strip obtained is used to produce, in two steps, a cup with a diameter of 73 mm, the plastic-coated side forming the outside of the cup. In the first step, a cup with a diameter of 100 mm is deep drawn from a round disc with a diameter of 150 mm. In the second step, this cup is formed into a cup having the final diameter of 73 mm by a further deep-drawing operation. This cup is fed to a wall ironing machine in which the wall thickness of the cup is reduced by wall ironing at a speed of 70 strokes per minute and using a single wall-ironing ring with an entry angle of 8xc2x0, which reduces the wall thickness of the cup by at least 40%. A sample is removed from the wall of the cup, whose wall thickness has been reduced by wall ironing, at a level of 50 mm from the base, in order to determine the crystallinity by X-ray diffraction. The crystallinity level found, as described above, must be greater than or equal to 20% in the samples prepared in this way.
Polyethylene terephthalate as mentioned above is understood to mean the polymerization product of a 50xe2x80x9450 mol % mixture of an acid comprising more than 99% terephthalic acid and an alcohol comprising more than 90% ethylene glycol.